Claude Sonnet is the strongest choice for tone-preserving document translation because it excels at following detailed style instructions and maintaining voice, register, and nuance across languages. You can paste your document along with explicit guidance about the desired tone and it will mirror those qualities closely in the Spanish output. ChatGPT is a solid alternative with similarly strong multilingual capability and instruction-following. Claude Opus edges ahead for especially complex or literary documents where the highest fidelity to style matters most. Gemini Pro is worth considering if you want a free-tier option with good Spanish fluency.